This invention relates to gage trimmers and, more specifically, to gage trimmers that are mounted on a bit gage for maintaining accurate tolerances in the bit gage area while drilling a hole in earth formations and to a bit incorporating the same.
Current gage trimmers 2, also referred to as xe2x80x9cpreflat cuttersxe2x80x9d, are manufactured from standard polycrystalline diamond cutters, which comprise a polycrystalline diamond layer (xe2x80x9cPCDxe2x80x9d) 4 over a carbide substrate body 5, as for example shown in FIG. 1. These gage trimmers are formed by cutting off a portion of a PCD cutter at an angle to the cutter central axis. This is typically accomplished by grinding to form a flat surface 6 extending across the cutter ultra hard material layer and a flat surface 7 across the body of the cutter as for example shown in FIG. 1. The two flat surfaces are typically angled relative to each other. The flat surface 6 formed on the ultra hard material layer is referred to herein as the xe2x80x9cultra hard material flatxe2x80x9d or xe2x80x9cultra hard material preflat areaxe2x80x9d. The ultra hard material flat extends from the upper surface of the PCD layer to the interface of the PCD layer with the substrate.
The gage trimmers 2 are mounted in the gage area 12 of a bit 14, i.e., the circumferential side area of the bit defining the bit gage (FIG. 2). The gage trimmers are typically mounted above all bit cutters and with their ultra hard material flat 6 parallel to the longitudinal axis of the bit, and thus parallel to the drill hole or bore wall 16 as for example shown in FIG. 1. Consequently the entire ultra hard material flat 6 is subject to engagement with the wall 16 of the drilled hole. For descriptive purposes the ultra hard material flat is also referred to herein as the xe2x80x9ccritical flat.xe2x80x9d The flat 7 formed on the gage trimmer body provides clearance between the trimmer body and the bore wall 16.
The critical flat 6 serves as a bearing against the drilled hole wall to prevent the wear of the bit body by the hole wall. Consequently, the gage trimmers do not wear as much as standard PCD cutters because they bear against and do not cut the bore wall. However, it is important that the gage trimmers do not wear significantly, otherwise the bit body can wear to a point that it is under gage.
One of the more significant problems with gage trimmers is that during drilling operations their PCD/carbide interface is exposed to significant shear loads. These loads are due to contact with hole walls by the critical flat during cutting. The gage trimmers are operated at an effective 0xc2x0 rake angle without any or with minimal clearance behind the ultra hard material, as for example shown in FIG. 1. Thus, in contrast with the normal full-round shear cutters, which are inserted into the bit at a rake angle, the gage trimmer interface between the PCD layer and the carbide body of a gage trimmer is subject to direct shear loading.
The interface region is often the weakest part of the gage trimmer. Consequently the direct shear loading on the interface region results in early failure of the gage trimmer. Furthermore, as the gage trimmer wears with time, the interface is exposed to abrasive wear and impact due to contact with the bore wall, which often leads to erosion of the carbide substrate, resulting in failure of the gage trimmer. Failure of the gage trimmers ultimately results in failure to keep the drill hole in gage.
A gage trimmer and a bit incorporating such a gage trimmer is provided. In an exemplary embodiment the gage trimmer comprises a body having a central longitudinal axis, a base and an end face, the body having a circumferential surface comprising a generally cylindrical portion and a relatively flat portion formed along a first plane. An ultra hard material layer is formed over the end face having a circumferential surface comprising a generally cylindrical portion and a relatively first flat portion formed along a first plane for bearing against a circumferential wall of a hole drilled by the bit. The first plane is inclined toward a diameter of the cutting layer in a direction away from the interface surface at an angle relative to a second plane parallel to a third plane aligned with said central longitudinal axis. The flat portion does not extend to the interface surface. Furthermore, in the exemplary embodiment, the ultra hard material layer thickness is greater at the circumferential flat portion than at the circumferential cylindrical portion of the ultra hard material layer.